!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

sys-fs/udev:0

(sys-fs/udev-171-r9::gentoo, installed) pulled in by
<sys-fs/udev-185 required by (net-wireless/bluez-4.99::gentoo, installed)

(sys-fs/udev-195::gentoo, ebuild scheduled for merge) pulled in by
>=sys-fs/udev-187 required by (sys-fs/udev-init-scripts-17-r1::gentoo, ebuild scheduled for merge)
(and 1 more with the same problem)

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously. If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

In this case, gnome brings in 2 packages: net-wireless/bluez-4.99 and sys-fs/udev-init-scripts-17-r1

net-wireless/bluez-4.99 requires a sys-fs/udev-185 version less than 185
sys-fs/udev-init-scripts-17-r1 requires =sys-fs/udev-187 r version at least 187

It is impossible to satisfy both constraints. Only the net-wireless/bluez-4.99 is negotiable. Either disable the "bluetooth" USE flag (assuming neither GNOME nor any of its dependencies have a hard dependency on it), or add "~net-wireless/bluez--4.101" to /etc/portage/packages.keywords

<gnome-base/gdm-2.91.94 is a "soft block". Portage will resolve that one for you.

For the other one <sys-apps/shadow-4.1.5-r1, it harder to tell, but it seems the fix for this is similar to before: Add "~sys-apps/shadow-4.1.5" to /etc/portage/package.keywords (note there is both an --r1 and -r2 in portage, this lien will get you -r2, but if you specifically want -r1, do "=sys-apps/shadow-4.1.5-r1" instead.

Note that shadow (and PAM) is critical to login - so have a boot CD handy in case something goes wrong and you lock yourself out!

Great. Without the build logs nto sure why its not compiling, but a compile problem with that package was just fixed today: https://bugs.gentoo.org/show_bug.cgi?id=445000. Resync and try again (or manually emerge the missing dependency gnome-common)